Yes, the Bonus Field is nothing more than a preview for the robot masters that are not complete yet and will be added to the game in a future expansion. Right now it's just a fun distraction where you can earn lots of experience points and earn some new abilities (like Shadownnico mentioned). And yes, Cossack mode will have the MM4 robots but the exact format of the game (like the order and number of missions) will be changed a great deal from what they are now. I'm so excited for it all to come together. :D